A hybrid drive system for a vehicle comprises a fuel powered heat engine and a battery powered electric motor, at least one of which is used to drive the vehicle. In the hybrid drive system, a battery provides electric power to the motor. The output of the battery must be maintained at sufficiently high level under varying environment conditions to meet power request demand on the motor. If the battery temperature drops, the internal resistance within the battery increases. This causes a drop in the level of the output of the battery. Accordingly, there exists a need to maintain the output of the battery at a sufficiently high level under varying temperature conditions.